A position of a concrete placement form for reinforced concrete structures is held by horizontal and vertical frame materials so that this form is not displaced and deformed by a pressure of concrete in a fluid state during concrete placement. Further, the frame materials are bound by a bundling material for strut materials and a linear material such as a chain which is a tension material so that the frame materials are not displaced even though vibration or a pressure during concrete placement is applied thereto. Furthermore, in shipping transportation, a container to be transported is fastened with a wire rope or the like without looseness so that it cannot be displaced. Tensile force of these linear materials, e.g., a chain or a wire rope is adjusted by a tension tool such as a turnbuckle so as to prevent slack from being produced.
The turnbuckle is a tension tool which has a screw thread, to which a treaded rod of a right screw is screwed from one end thereof and a threaded rod of a left screw is screwed from the other end thereof, is formed at each of both ends of a body frame, and which moves positions of the threaded rods at both the ends closer to or away from each other by rotating the body frame. A locking portion such as a hook is formed at an end portion of each threaded rod, and the locking portion is locked with respect to a chain or a wire as a linear material whose tensile force is adjusted. The turnbuckle has an advantage that a length for pulling in the hooks at both the ends closer can be increased, whereas it has a problem that the number of times of rotation of the turnbuckle is increased and tensile force adjustment takes time and labor when the length for pulling in is long and that the body frame is close to the linear material and hence the body frame is hard to rotate when the linear material is not loose enough.
Thus, Japanese Unexamined Patent Application Publication No. 1992-300408 discloses the technology by which protruding portions are formed at a plurality of positions on an inner shaft along a body frame, notch portions are formed in an engagement plate placed at an end portion of an outer cylinder of the body frame, the inner shaft is axially slid along the body frame, the protruding portions are engaged with the notch portions, whereby a length between respective coupling tools formed at the end portion of the body frame and the end portion of the inner shaft can be adjusted. According to this technology, the length between the coupling tools at both the ends of the turnbuckle can be easily adjusted, whereas there is a problem that the number of components constituting the turnbuckle increases.
Further, Japanese Unexamined Patent Application Publication No. 2006-070572 discloses the technology of an adjustment tool that has hook portions on both a head side of a bolt and a distal end side of a shaft body of the bolt, attaches hooked cylindrical bodies freely fitted onto the shaft body, and fastens a nut, and thereby adjusts a distance between the two hooked cylindrical bodies to eliminate slack of a chain or the like for hanging a work platform.
According to this technology, since the pair of hooks are secured to lateral sides on the bolt head side and the bolt distal end side, and the bolt or the like cannot enter between the pair of upper and lower hooks. Furthermore, when the pair of hooks are arranged to face each other, the pair of hooks can be held in an opposed state, the hook on the bolt head side can be placed on the upper side, the hook on the bolt head side and the bolt shaft portion can be simultaneously held with one hand, the hook can be easily engaged with an upper link of the chain, and the hook on the bolt end side can be easily engaged with a lower link of the chain.
However, in case of a chain stretched with less slack, at the time of engaging the hook on the bolt head side with the upper link, the hook on the bolt distal end side that is in the freely fitted state interferes with the other link and rotates, and hence the hook is hard to be hooked. Moreover, after the hook on the bolt head side is hooked, the hook on the bolt distal end side must be held with one hand, a position of the link of the chain placed on the bolt distal end side must be adjusted with the other hand while sliding the bolt shaft along the cylindrical body on the bolt head side, the hook on the bolt end side must be fitted to the link while paying attention to prevent the hook on the bolt head side which has been hooked before the counterpart from coming off the link, and hence there is a problem that disposing the coupling tool is difficult.